Impact of Covid-19 pandemic lockdown on the urban litter and clean environment index

Changing the level of pollution in the urban environment is one of the consequences of Covid-19. Litter are one of the most important urban pollutants affected by the Covid-19 pandemic. In this research, the pollution level of urban areas during the Covid-19 pandemic was investigated by studying the urban environment. To this end, the protocol of observation and counting was used and litter were studied in two groups including common litter and Covid-19 related litter in Yasuj, Iran. The results were interpreted based on the clean environment index (CEI). The time of observation was selected based on the peak of the disease and the decline in the incidence rate. The results showed that on average, at the peak of the disease, the density of the litter was reduced by 19% compared to the low lockdown related to Covid-19. The CEI on average was 4.76 at the peak of the disease that was interpreted in the clean status, while the CEI on average was 5.94 at the low lockdown related to Covid-19 so interpreted in the moderate status. Among urban land uses, recreational areas with a difference of more than 60% showed the greatest impact caused by Covid-19, while in commercial areas this difference was less than 3%. The effect of Covid-19 related litter on the calculated index was 73% in the worst case and 0.8% in the lowest case. Although Covid-19 decreased the number of litter in urban areas, the emergence of Covid-19 lockdown related litter was a cause for concern and led to increasing the CEI.

Litter study protocol. Information on the density of litter (item/m 2 ) was obtained based on the field method for counting the number of litter using visual surveys 18,25 . In this method, the studied locations were visited by the researchers at certain times of the day, and information was obtained by direct observations 11,19 . The study was based on a specific protocol in the evening of working days 18,26 . The criteria for the investigation of the studied locations was only the number of litter, and the weight and volume of the littered wastes were omitted due to the impact of secondary pollution and intervening conditions such as moisture 27 . The streets were surveyed on both sides. The total width of the sidewalk plus a meter of street depth was determined as the width of the liter count 18,28 . The study was conducted in one year (September 2021-September 2022), during which time each location was visited 12 times (once every month) and data were recorded. At the time of this study, due to the increase in the number of positive cases of Covid-19 and the rate of hospitalizations, the government implemented lockdown twice, which included restrictions on the presence of citizens in the city and reducing the hours of commercial and administrative activities. The first and second lockdown periods lasted about three weeks and four weeks, respectively. According to the one-month interval of litter assessment in each location, two assessments out of a total of twelve assessments were conducted for each location during lockdown.
Litter study criteria. Littered solid wastes by citizens in the environment was considered litter and items such as tree branches, leaves and broken pieces of sidewalk surface were not considered 23 . In this study, litter were investigated in two general categories, which included common litter (CL) and COVID-related litter (CRL). The CL included items such as plastics, papers, metals, wood, and tobacco wastes. Items such as face masks, gloves, alcoholic solution-based bottles, and face shields were classified as CRL.
Calculation of pollution status. Information on the quantity of litter at the studied locations was interpreted using the clean environment index (CEI) 29 .  www.nature.com/scientificreports/ In this formula Ni represents the number of observed litter and K is a constant coefficient equal to 20. The Wi coefficient used in this formula is defined according to the potential for damage of each litter to the environment and health 18,29 . The width in this formula for each location included the entire width of the sidewalk and 1 m from the street. The width in this formula for each location included the length of the street 30 . The Wi coefficient for different categories of litter has been shown in Fig. 2. Some types of CRL were calculated with Wi = 2.5 in the formula due to the possibility of virus infection, such as face masks. But some types of CRL, which are made of plastic and have a low probability of being infected with the virus, were calculated with Wi = 1.5 in the formula 28,29 .

Results and discussion
The density of litter at the locations studied have been shown in Table 1. The average of observed litter in two lockdown times is stated in the LT section. The average amount of observed litter in ten times without lockdown is stated in the NLT section. Among the locations, the litter density in LG was 0.689 item/m 2 , which was higher than that in other locations. Also, the litter density was at the lowest value equal to 0.0125 item/m 2 , which was observed in LJ and was 54 times lower than the highest litter density in the studied areas. On average, in the areas studied, the litter density was 0.287 items/m 2 . A cigarette butt was the most CL in the urban environment, with an average of 0.150 items/m 2 , consisting of 52.26% of the total urban litter. The CRL in the total studied locations consisted of 0.88-62% of the total litter, which was equal to 0.0011-0.0069 item/m 2 . On average, CRL accounted for 1.49% of total litter.  www.nature.com/scientificreports/ During this study, the two times COVID-related lockdown were implemented by the government, and the results of the litter assessment in the areas studied in these two periods showed that the litter density was reduced by an average of 19.86%. As shown in Fig. 3, the CRL density at the time of the lockdown was 0.001-0.0064 item/ m 2 , while the density of CL was 0.0036-0.6060 item/m 2 . These conditions showed that during the COVID-related lockdown, the density of CL in the urban environment decreased by an average of 19.92%. The proportion of CRL in the total litter at this time was 1.63%, but under normal conditions was 1.48%. Therefore, spatial variation of litter density can be observed in the studied areas. Of course, during the pandemic, the trend of changes in the amount of litter is different from household waste. During the pandemic, due to the increase in online shopping, the amount of plastic in household waste increases 10 , but as shown in Fig. 3 owing to the decrease in the presence of people in public places and the outdoor environment, the amount of litter compounds decreases, but the proportion of plastic in them increases due to the increased use of face masks 29 .
Other studies have reported that litter is a major pollutant in many urban areas. In a city in Argentina, the density of litter was studied in four different areas, which showed that there was different litter densities in all areas and cigarette butt was the most CL 23 . In our study, the average density of cigarette butt was 0.153 number/ m 2 , which was the highest CL. However, the composition of the litter in our study was different from other cities, so the highest numbers of litter were cigarette butts, facial tissue, paper, and plastic, respectively. One of the most important reasons for the difference in litter density in different urban areas can be the difference in land use 18 . In this study, the highest litter density was seen in commercial areas in LA, LD, and LG, while the recreational areas including LC, LH, and LJ had the lowest litter density. This difference was particularly noticeable in the case of CRL. As mentioned in Table 1, the highest CRL was observed in LC and LH, which were places of recreational areas, while the lowest density of CRL was observed in residential areas, including LB, LF, and LK. Differences in litter density in different land-uses mentioned in the results of previous studies 18,29,31 . One of the most important reasons for spatial variation is the impact of land use on population density 32 . In commercial areas, due to the higher people density, waste littering by citizens are more probably, and therefore the litter density in these areas will be higher than in other uses such as residential areas 23 . Moreover, some structural conditions in urban environments can cause more durability of the litter and thus increase the litter density in the environment. For example, the presence of low-access points such as tree pits, surface water collection canals, and bicycle stations reduces cleaning efficiency and increases litter density 31 . This situation was also observed in the case of CRL so that higher population density in commercial areas was one of the reasons for the higher density of CRL in these areas.
Littering potential of some types of municipal solid waste at specific points another reason for spatial variation is the litter density in the urban environment. For example, littered cigarette butts are more found around cigarette sales and consumption centers 20 and more littered paper receipts are observed around banks and ATMs 18 . The difference in the number of these points in different parts of the city causes the variation of litter density. However, the results showed that the CRL density did not depend on this factor and no specific points were observed for higher density of this type of litter. The density of some types of litter such as cigarette butts around stalls and supermarkets and paper receipts around ATMs was higher than that in other areas. Also, the density of litter was not the same in the locations studied. Places, where people may stop for a while, have the potential for more litter density, the most important of which are intersections and urban transportation stations 18,25 . Differences in the quality of cleanup in different places can also be considered as a reason for spatial variation of litter density 32 . In this study, recreational areas had a different cleanup process from other places and therefore the litter density in them was different from other areas.
The interpretation of urban environmental pollution status based on CEI is shown in Fig. 4. The results showed that 25% of the studied areas were in very clean status and 50% were in dirty and extremely dirty status. www.nature.com/scientificreports/ The average index for the studied areas was 9.72 and showed a moderate status. The use of CEI showed that this index shows the conditions in a more appropriate way compared to the litter density. Applying a coefficient for each litter and considering the importance of each litter in terms of pollution emission or environmental risk ranked the impact of each dump in the index. Among the litter, cigarette butt had the most important effect on CEI due to their large number and coefficient equal to 2. This index covers the effect of the type of litter, but it does not reflect the amount of pollutant leakage from different litter in different climatic conditions 30 . Due to the fact that cigarette butts contain various pollutants such as toxins 33 , metals 34 , and organic compounds 35 , a coefficient of 2 was applied for it. One of the important features of cigarette butts is owing to their rapid pollutant leakage into the environment, which makes this litter more important than other types. For example, it has been reported that nicotine leaks quickly from cigarette butts, and, in turn, the leaked nicotine can pollute one cubic meter of water 31 . However, in this study, CRL was another important type of liter that had a significant impact on CEI. This type of litter is made of plastic-based materials and is known as a source for microplastic 36 as well as the possibility of virus infection. Hence, CRL was applied in the index by a coefficient equal to 2.5, which was the highest coefficient among the types of litter. As Table 2 shows, the impact of each type of litter on CEI was different for the locations studied. This effect was dependent on the quantity of litter and the coefficient of each litter.
CRL had an independent effect of 0.56-62% on the density of total litter in different locations in the urban environments, as well as the impact of these litter on the CEI of the areas studied was 1.2-73%, which averaged 18.34% (Fig. 5). A comparison of the periods with COVID-related lockdown with other days of the year showed that the pandemic led to change in litter composition in the studied locations on average (see Fig. 3). However, the significant impact of CRL on CEI showed that the pandemic due to the emergence of new types of litter in the urban environments resulted in an increase of 0.05-0.34 in the index score for the different locations studied. For this reason, the pandemic increased the pollution index score by an average of 0.21 point (the average ratio of CRL in the CEI were calculated for the studied locations as shown in Fig. 4).
Although the COVID-19 pandemic changed the density of litter in the urban environment and led to the emergence of new types of litter that can affect the landscape of the city, a more serious consequence of the pandemic to the urban environment is the possibility of damage due to degradation of CRL. Face masks and gloves were the two main CRLs which were observed in this study that constituted 1.5% of the total litter composition in the urban environments and were effective in the pollution index on average 18%. According to reports, in Iran, consumption of face masks and gloves during the COVID-19 pandemic was increased 55 times and 2.5 times, respectively 37 . The generation of waste from the consumption of this equipment, part of which is littered in cities, is a source of different kinds of microplastic in the environment that is a serious concern 38 . On the  www.nature.com/scientificreports/ other hand, according to health protocols, the collection and disposal of these COVID-related wastes should be in separate bags 37 , but their littering in public places, as well as disposal with other municipal waste, cause the risk of infection transmission. This is especially the case for informal waste management staff in developing countries because most of these people do not use personal protective equipment during direct contact with municipal solid waste 39 .
In Iran, the management of medical wastes is the responsibility of its producer. For this reason, health centers and hospitals were equipped with disinfection devices such as autoclaves, which were able to manage well the COVID-related wastes during the pandemic 16 . However, in the case of CRL, a separate management system was not foreseen 37 and the results of this study can be used in making decisions to improve the management of CRL. Given the impact of citizens' behavior on littering solid wastes in public places, efforts to improve citizens' behavior can be effective in reducing CRL 29 . This is especially important for face masks and gloves because there are special protocols for disposing of this waste during the pandemic 37 . One of the reasons for the presence of litter in public areas such as beaches can be the lack of trash 18 , however, in the urban areas studied in this study, the presence of many trash bins, a significant density of litter was seen. The situation in the areas studied showed that in the continuation of the COVID-19 pandemic and in probably similar situations in the future, the CRL should be better managed. In general, litter control in urban environments can be done in three phases: prevention, mitigation, and removal 21,40 . In the prevention steps by modifying the behavior of citizens through education and also applying anti-littering laws, litter can be reduced, including CRL. The most important action in the mitigation step is to reduce the density of litter, including CRL, the installation of trash bins in public places, as well as the installation of containers for CRL at specific intervals. Finally, the last step is to improve the cleanup efficiency of the urban environment, especially by identifying low access points in the removal step. Improving the management of litter can reduce the adverse consequences of landfilling, such as the risk of disease transmission 41,42 .

Limitations and strengths of study
In this study, all land-uses were investigated and several locations from each land-use were studied. This was a field study and the data were obtained directly from the urban environment and represented the reality. A new index was used to interpret the data and the impact of the Covid-related litter was surveyed in this index. But this study had limitations such as the impossibility of considering all the streets of the city. Also, there was not enough time to study other cities and compare the results. Moreover, investigation the impact of litter on the pollution of water and soil resources and interpreting it by the new index was another drawback of the current research, which can be considered in future research.

Conclusion
The effect of the COVID-19 pandemic on litter and urban pollution was investigated. The results showed that the COVID-related litter accounted for an average of 1.49% of the total solid wastes. Cigarette butt was the most common waste, accounting for 51.5% of the urban wastes. Also, 50% of the areas studied with a score of 10 and higher were in a dirty and worse status considering the CEI. Furthermore, 25% of the studied locations were in a very clean status considering the clean environment index. The COVID-related litter had a 1.2-73% effect on the pollution index, increasing the CEI by an average of 0.21 points. Although the COVID-19 pandemic reduced population density in public places and reduced the number of litter due to lockdown, it led to the emergence of new types of litter, resulting in an increase in the pollution index in the urban environments. www.nature.com/scientificreports/

Data availability
All data generated or analyzed during this study are included in this published article.